Specific Bt toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton. The choice of genes depends upon the crop and the targeted pest, as most Bt toxins are insect-group specific.

The toxin is coded by a gene name cry. There are a number of them, for example, the proteins encoded by the genes cryIAc and cryIIAb control the cotton ballworms, and that of cryIAb controls corn borer.